An example of a known glass ceramic substrate is a laminated, multilayer ceramic substrate, which contains a stack of glass ceramic layers. A laminated glass ceramic substrate contains various wiring conductors, including those that form passive elements such as capacitors and inductors, and those used to establish connections such as electrical connections between elements.
Examples of wiring conductors placed in laminated glass ceramic substrates include internal electrodes, which are formed along the interfaces between the glass ceramic layers, and via-hole conductors, which are formed through the glass ceramic layers to provide interlayer connections.
There are also wiring conductors formed on the surface of a glass ceramic substrate, or more specifically, on the outward surface of the outermost glass ceramic layers. An example is a surface electrode. Some surface electrodes are used to couple the glass ceramic substrate to the electronic components to be placed on the surface of the substrate, and some others are used to couple the glass ceramic substrate to a motherboard. Examples of electronic components that are placed on the surface of a glass ceramic substrate include those having bumps, those to be brought into a wire-bonding process, and those having planar terminal electrodes to be soldered.
Such a surface electrode is usually formed by applying a conductive paste, which is prepared by dispersing a conductive metal powder in an organic vehicle, and then firing the applied paste. There are two ways of firing the applied conductive paste. In one, which is referred to as cofiring, the conductive paste is applied to an unfired glass ceramic layer and fired together with the glass ceramic layer. In another, the conductive paste is applied to a sintered glass ceramic layer and fired alone.
In both cases, insufficient bonding of the surface electrode to the glass ceramic layer causes the electrode to peel off. The cofiring method described above can lead to improved bonding strength because the glass component of the glass ceramic layer penetrates into the surface electrode during the firing process.
Another example of a technology that can improve the bonding strength of a surface electrode is that described in Japanese Unexamined Patent Application Publication No. 2000-173346 (Patent Document 1). Patent Document 1 discloses a conductive paste that firmly adheres to BaO—Al2O2—SiO2 ceramic layers. The conductive paste described in Patent Document 1 contains a Cu powder, a ceramic powder, and an organic vehicle. Some requirements are specified for the ceramic powder. For example, (a) the ceramic powder is at least one selected from the group consisting of Al2O2, SiO2, and BaO, (b) has an average particle diameter of 0.1 to 3.5 μm, and (c) constitutes 1% to 15% by volume based on the volume of the Cu powder.
According to Patent Document 1, forming a surface electrode on a ceramic layer by using this conductive paste imparts improved bonding strength to the surface electrode because during the firing process, the glass component synthesized in the ceramic layer is drawn up into the surface electrode by the action of the ceramic powder, giving the interface between the ceramic layer and the surface electrode an irregular shape, producing an anchor effect.
In general, a surface electrode formed by a printing process (e.g., screen printing) using a conductive paste tends to be thinner in the edge portion in a plan view than in the middle portion because of the effects of printing defects such as running and bleeding of the printed paste. Thus, the use of the conductive paste described in Patent Document 1 to form a surface electrode may result in reduced conductivity of the surface electrode on its surface because the glass component drawn up from the ceramic layer into the surface electrode is likely to reach the surface of the electrode and get exposed particularly in the edge portion in a plan view, the portion where the surface electrode is relatively thin.
A surface electrode obtained by firing a conductive paste is usually coated with a plating layer. For example, a surface electrode obtained by firing a conductive paste containing Cu as a conductor is coated with a plating layer such as one composed of a Ni plating film and an Au plating film formed on it. If the glass component is exposed on the surface of the surface electrode as described above, however, reduced platability of the surface electrode makes the plating layer more likely to peel off of it. In particular, reduced platability of the surface electrode in the edge portion in a plan view leads to a significantly low peeling resistance.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2000-173346